1. Field of the Invention
The present invention generally relates to Global Navigation Satellite System (GNSS) signals and, more particularly, to signal decomposition for direct and indirect rays present in the GNSS data.
2. Description of the Related Art
Multipath is typically a major source of error in contemporary GNSS systems. Ray models are generally used to describe GPS signals that are received in the presence of multipath propagation effects and white Gaussian noise. Specular reflections in particular are typically described using a ray model. A Multipath Estimating Delay Lock Loop (MEDLL) receiver is a prominent example of a receiver design employed specifically to mitigate the effects of multipath. The MEDLL receiver makes explicit use of multipath parameter estimation and tracking to reduce multipath effects on receiver outputs. MEDLL works by simultaneously estimating parameters for both line-of-sight and multipath signals. There are simultaneously operating tracking loops employed to track each of the signals present, both line-of-sight and multipath.
The mathematics in MEDLL has been described to be akin to performing nonlinear curve fits. A set of reference correlation vectors are generated, each of which has an amplitude, phase, and propagation delay that would be associated with a signal present in received data. Maximum likelihood estimation is then used to obtain multipath signal parameter estimates that minimize the error between received signals and estimated signals generated by the receiver. The number of distinct signals generated within the receiver can be varied, depending on how many multipath signals are detected. MEDLL is perhaps the most prevalent multipath mitigation technique that can be found in GNSS literature in which multipath estimates are explicitly obtained. However, a disadvantage of the conventional MEDLL receiver is a feedback loop, which may improperly bias future loop outputs based on past results that have been erroneously obtained.
Commonly used multipath models do not adequately represent highly complex GNSS signal environments encountered in urban or indoor environments or under foliage. Because of this, a mitigation technique has been developed and verified in the lab, only to be deployed for use unsuccessfully because of the model's lack of adherence to the characteristics of the true environment. This problem may be alleviated through the use of a recording and playback system. A system such as this is used to record real-world GNSS signals, and then replay these recorded signals. This is distinct from a simulator that uses models to construct signals that demonstrate propagation effects that would be present in the channel. The advantage of a recording and playback system is that there is no uncertainty regarding the validity of the effects observed in the recorded signal. The disadvantage of this system is that no models are obtained that can be used in mathematically describing the channel.
Accordingly, there is a need in the art for a modeling system that offers both the advantages of a recording and playback system and a system that uses an analytic model to generate data without the feedback effects seen in a MEDLL receiver.